Django and Memcached: Set Value Not Working

I had a strange memcached bug while using Django. Sometime cache.set(key, value) would save the value. Other times it would not. No warnings or errors. Just not saved.

It turns out the item I was trying to save was near the item size limit of memcached. The default is 1 MB (see the -I flag). Sometimes the item would be less than the max and set would work. Other times it would exceed the limit and set would just stop working.

The man page recommends not exceeding the default. So I solved the problem by saving the item in two parts.

I should also mention that Django file-based cache is pretty easy to use and does not have this limitation. Since all my Django sites run on SSD drives, file-based cache is almost as fast as memcached.


Django, Selenium Headless Tests Fail

Running headless selenium tests is great. Except when the tests succeed when headed, then fail when headless. When that happens it’s hard not to let your imagination run wild with crazy javascript thoughts.

There are many ways to run headless. The method I use is described in this post. In a sense this is not headless because a normal browser is running; it’s just running on a hidden display. This makes the headless fails all the more perplexing.

In this case, the problem was the headless browser window was a different size compared to the headed version. I use the chosen widget a lot. It’s a fancy combo of an HTML select widget and auto-complete. Clicking on this widget creates a pop-up drop down. If the needed choice is outside of the browser, selenium auto scrolls the page so that the choice is visible. When it decides to scroll depends on the browser window dimensions.

In many cases, this auto scroll would no create problems. In my case, I also have a nav-bar locked to the top of the screen. Sometimes the desired choice would end up under the nav-bar after auto-scroll. When selenium moved to select the choice, it would get the nav bar instead. The form value would not be set and the form would fail. Although it did not happen to me, I supposed it’s possible selenium could have ended up on a different page. That could make some strange errors.

The first part of the fix is to explicitly set the browser window so the headed and headless versions are the same. Something like this:

if getattr(settings, 'HEADLESS_TESTS', False):
    self.vdisplay = Display(visible=False, size=(1600, 1000))
self.selenium = webdriver.Firefox()
self.selenium.set_window_size(1500, 900)

If all the problems go away, you are done. If not, then hopefully the headed tests and headless tests will now fail in the same way, making debugging easier.

In my case, I added code to scroll the page if the item was under the nav bar. Here is the code.

Django Include Template Tag Stops Working

The Django template tag {% include %} worked fine with DEBUG=True, but with DEBUG=False, the page rendered fine, without errors and without the content of the include.

Here’s what was happening:

{% load staticfiles %}

<link rel="shortcut icon" href="{% static 'images/favicon/favicon.ico' %}">
<link rel="icon" href="{% static 'images/favicon/nonexistent_file.png' %}">

Evidently, when staticfiles fails, the entire content of the include is tossed out. In my code, the first link was valid, but the second was not. I expected at least the link to be rendered.